Dogs do not have a clear notion of time, but they know what time you should return from work, what time he would go out for the daily walks and what time other activities are supposed to take place. The reason behind this is the internal biological clock of the dog. He is used to doing different things at certain times during the day so he will act according to his inner instincts.
Other aspect which needs to be considered is that we humans are very predictable. We wake up around the same hour, eat at certain hours during the day, take dogs out for walks more or less at the same hours, so a dog will know based on habit when things are supposed to happen and will act accordingly.
Dogs perceive the smallest changes in your routine so usual separation problems occur when there are changes in your daily routine which will affect him and will disturb his daily schedule.
I would not bother very much about the 8 hours he has to spend alone, if you walk him enough (at least 30min) in the morning, give him something to eat/chew when you leave for work. He will spend most of the time you are gone sleeping and therefore will not notice your absence.